The present invention relates to an optical system employed in a photographing device having a beam splitting element for splitting light incident thereon, and in particular, to an optical system employed in a photographing device such as a single-lens reflex camera and/or an electronic still camera, in which light from an object is entered through a photographing lens, and then is split by the beam splitting element and directed towards a finder optical system and to a film plane of a image receiving surface.
Such optical systems used in, for example, in electronic still cameras are generally constituted such that a beam splitter splits light from an object, which is entered through a part of a photographing lens system, into a beam directed to a photographing CCD image sensor and a beam directed to a finder optical system.
The beam splitter used in such an electronic still camera has a beam splitting surface. A part of the light incident from the photographing lens is reflected by the beam splitting surface and directed to the finder optical system, and the remainder part of the light which passes through the beam splitting surface is directed to the CCD sensor.
In such a optical system, however, light entered from an eyepiece lens may disturb an object image formed on the image pickup surface of the CCD sensor. If light enters through the eyepiece lens of the finder optical system, the light may be incident onto the beam splitting surface and may pass through the beam splitting surface. The light passed through the beam splitting surface may be reflected on the end surface of the beam splitter, and is internally reflected thereby. Then, the reflected light is further reflected by an opposite side surface of the beam splitting surface, and may be incident on the CCD image sensor as a ghost light. It should be noted that the term "ghost light" refers to light which disturbs the image formed by the photographing optical system, i.e., the light which affects the image formed on the CCD image sensor, or image receiving surface. In addition to the above problem, due to recent tendency of increased magnification of the eyepiece lens of the finder system as the size of the CCD image sensor becomes smaller, intensity of the ghost light which may disturb the image formed on the CCD image sensor is increased.